The present invention relates to a variable displacement gear pump.
A gear pump has a drive gear and a driven gear engaged with each other to raise a pressure and transferring fluid out of the pump. If the fluid to be pumped by the gear pump is a hydraulic fluid, the gear pump can actuate various hydraulic devices provided in a hydraulic circuit. The gear pump is simple in structure, easy to operate and maintain and low in cost in comparison with other types of pump. Additionally, the gear pump is hardly influenced by foreign matters contained in fluid, and suitable for reduction in size and weight. Thus, the gear pump is used advantageously, for example, for a hydraulic fluid pump driven by an internal combustion engine or an electric motor of an industrial vehicle such as a forklift truck.
The displacement of a gear pump is determined by its rotational speed and, therefore, it is difficult to change the displacement of the gear pump without consideration of the rotational speed of the gear pump. Excessive displacement will force the gear pump to do extra work. Therefore, a variable displacement gear pump has been proposed which changes the pump displacement by using a plurality of gear mechanisms. In this variable displacement gear pump, changing of the displacement is accomplished by changing between two mode operations. In one mode operation, a specific gear mechanism is used to pump and discharge fluid, and in the other mode operation, the pumped fluid is returned from the gear mechanism to an inlet port of the gear pump.
The Japanese Patent Application Publication No. 2002-70757 discloses a variable displacement gear pump having in its casing a drive gear and two driven gears engaged with the drive gear thereby to form a main body of the gear pump. The main body of the gear pump has two pump lines including a first pump and a second pump, which functions as a double gear pump. Outlet and inlet ports of the second gear pump are connected with each other through an unload passage having therein an electromagnetic opening valve. When the electromagnetic opening valve is closed, the first and second pumps are operated in parallel, thereby increasing the displacement of the gear pump. During this operation, the gear pump is operating at a large displacement. When the electromagnetic opening valve is opened, the second pump is unloaded thereby to decrease the displacement of the gear pump. During this operation, the gear pump is operating at a small displacement.
In this type of variable displacement gear pump, the first and second pumps are arranged parallel to each other. Because of the rotational direction of the drive shaft, inlet and outlet ports of the first pump are located opposite to the inlet and outlet ports of the second pump, respectively. That is, the inlet port of the first pump and the outlet port of the second pump are located on one side of the gear pump, while the outlet port of the first pump and the inlet port of the second pump are located on the other side of the gear pump. The variable displacement gear pump has suction and discharge passages formed by merging passages on the suction and discharge sides of the first and second pumps, respectively.
In the variable displacement gear pump disclosed in Japanese Patent Application Publication 2002-70757, if the unload passage ensures sufficient flow rate for unloading, the unload passage must be formed with a large cross-section. Thus, an electromagnetic valve to be provided in the unload passage will become inevitably larger in size as the cross-section of the unload passage is increased.
The present invention which has been made in light of the above problems is directed to providing a variable displacement gear pump. The variable displacement gear pump is operated without using an electromagnetic valve serving as an opening valve in a bypass passage through which hydraulic fluid discharged to a discharge-side space of a sub gear pump portion is returned to a suction passage.